A conventional method for correcting the refractive error in a cornea is keratophakia, i.e., implantation of a lens inside the cornea. Keratophakia uses an implant which is placed into the cornea approximately equidistant from the exterior surface of the cornea and the interior surface. The procedure is usually done by first preparing a lens from corneal donor tissue or synthetic material using a cryo-lathe. The lens is implanted by removing a portion of the cornea with a device called a microkeratome, and the tissue is sutured back into place over the lens. However, there can be problems when microkeratomies are used for cutting the cornea. First, irregular keratectomies or perforations of the eye can result. Second, the recovery of vision can be rather prolonged.
Another surgical technique exists that uses a femtosecond laser to separate layers inside the stromal, at least two-thirds of the distance from the top surface of the cornea to the inside of the eye. An incision is made to access this area and a solid inlay is inserted to help correct myopia in the eye. By separating the layers in the bottom two-thirds of the stromal, it is difficult to access the separated area to insert the inlay and virtually impossible to change or modify the inlay without another extensive surgical procedure. This procedure requires making an incision which is parallel to the visual axis and is limited in the lateral direction by a maximum size of 0.3 mm to encase a relatively rigid inlay that forces the tissue in the lateral direction.
Additional surgical techniques exist that use ultraviolet light and short wavelength lasers to modify the shape of the cornea. For example, excimer lasers, such as those described in U.S. Pat. No. 4,840,175 to Peyman, which emit pulsed ultraviolet radiation, can be used to decompose or photoablate tissue in the live cornea so as to reshape the cornea.
Specifically, the Peyman patent discloses the laser surgical technique known as laser in situ keratomycosis (LASIK). In this technique, a portion of the front of the live cornea can be cut away in the form of a flap having a thickness of about 160 microns. This cut portion is removed from the live cornea to expose an inner surface of the cornea. A laser beam is then directed onto the exposed inner surface to ablate a desired amount of the inner surface up to 150-180 microns deep. The cut portion is reattached over the ablated portion of the cornea and assumes a shape conforming to that of the ablated portion. Additionally, in the Lasik procedure, a femtosecond laser can be used to cut and separate the flap.
However, because only certain amount of cornea can be ablated without the remaining cornea becoming unstable or experiencing outbulging (eklasisa), this technique is not especially effective in correcting very high myopia. That is, a typical cornea is on average about 500 microns thick. The laser ablation technique requires that at least about 250 microns of the corneal stroma remain after the ablation is completed so that instability and outbulging do not occur.
Additional methods for correcting the refractive error in the eye include inserting an implant in-between layers of the cornea. Generally, this is achieved using several different methods. The first method involves inserting a ring between layers of the cornea, as described in U.S. Pat. No. 5,405,384 to Silvestrini. Typically, a dissector is inserted in the cornea and forms a channel therein. Once it is removed, a ring is then inserted into the channel to alter the curvature of the cornea. In the second method, a flap can be created similarly to the LASIK procedure and a lens can be inserted under the flap, as described in U.S. Pat. No. 6,102,946 to Nigam. The third method involves forming a pocket using an instrument, and inserting an implant into the pocket, as described in U.S. Pat. No. 4,655,774 to Choyce.
However, with the above described techniques, a knife or other mechanical instrument is generally used to form the channel, flap or pocket. Use of these instruments may result in damage or imprecision in the cut or formation of the desired area in which the implant is placed.
Therefore, there exists a need for an improved method of correcting refractive error in the cornea of an eye.